This also seems like an easy way for VPN providers to differentiate themselves with their apps. The fact that it hasn’t happened makes me think that it’s impossible with unrooted iOS
That domain could constantly resolve to different IPs, requiring updates to the routing rules, and those IPs could be shared with many other domain names that the user didn't list (for example Cloudflare IPs). So the mapping isn't clean and you're likely to miss some IPs some of the time or incorrectly intercept some traffic that the user didn't want to route through the VPN.
A proxy would not have this problem, it gets to inspect the request and hostname and then decide how to reach that host.